Analytical columns packed with particles or microparticulate packings are used in many chromatography instruments for the separation of components. In their optimum design, packed chromatography columns have a uniform packed bed with no cracks or channels, and without sizing or segmentation of the particles within the column. The particles are packed as densely as possible without being fractured during the packing process. Relatively large particles, such as those in the range of 30-40 .mu.m, are typically poured into a column in a dry state. This is known in the prior art as dry-packing. Although dry-packing techniques have been used for some time, columns of high separation efficiency are more difficult to produce as particle size decreases. This is due to the fact that smaller particles have high surface energies relative to their mass, and hence tend to clump or algolorate. Such particle agglomeration causes non-uniform compaction during the packing process, which results in widely varying flow velocities along the columns channeling and thus poor column efficiency.
It is known in the prior art to use high pressure "wet fill" or slurry packing techniques to pack particles having a diameter less than 20 .mu.m, because these small particles are difficult to form into high efficiency columns by dry-filling. In the slurry technique, a suitable liquid wets the particles and eliminates particle aggregation during packing. Proper selection of the liquid reduces the tendency of the particles to size-fractionate via gravitational sedimentation. Particles settle at a rate which is in proportion to the square of their radius and to the difference between their density and the liquid's density. This indicates that large particles settle faster than small particles. The strong dependence of settling velocity in particle size means that the wider the distribution of particle sizes, the more quickly an initially homogenous slurry of particles will become heterogenous during handling.
Thus, it is known in the prior art to use a suspending fluid that has a density equal to that of the particles. This approach is called "balanced density slurry-packing". Typically, various liquids having specific densities must be mixed in proportion to match the density of the packing particles. Once a liquid having the appropriate density is attained, it is mixed with particles to form a suspension. This suspension is then pressurized with a gas to force it into the column. The liquid is consequently drained off. This technique produces efficient columns but suffers several restrictive drawbacks. Firstly, the liquid medium typically must be produced by mixing a variety of liquids in proportion to match the specific density of the particles. This mixing and matching has been known to be a tedious process. Secondly, the liquid medium must also be matched for polarity with the particles so that aggregation due to electrical attraction and repulsion is prohibited and to prevent chemical reaction of the particles. Therefore, the liquid medium must be mixed for correct polarity. This further increases the complexity of forming the appropriate liquid medium. Thirdly, the liquid medium once matched for density and polarity is forced into the column with a pressurized gas, adding even another medium which must be in contact with the particles during packing.
The present invention offers an improved slurry-packing technique in which the suspending medium is (originally) a gas which is pressurized until its density matches that of the particles. Therefore, there is no need for mixing and matching of liquids. Further, the suspending medium does not need to be forced into the column with another pressurized medium, since the "gas" is already pressurized.